Method of detecting the defects in a dielectric coating at the surface of an electrically conductive underlayer

ABSTRACT

The method consists in placing a conductive underlayer (1) coated by a dielectric layer (5) in a conductive medium in which an excitation electrode (2) is positioned at a distance from this underlayer (1). This underlayer (1) and this excitation electrode (2) are being connected to the corresponding terminals of an alternative-current generator (4) and between this electrode (2) and this underlayer (1) one interposes a catcher coil (6) oriented in a plan perpendicular to the electric field lines and a potential-measuring electrode (7) in the opening of said coil (6) and one determines the impedance resulting from the voltages and currents detected.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a national phase application ofPCT/CH82/00109 filed Oct. 11, 1982 and based in turn upon a Swissnational application No. 6619/81-9 filed Oct. 16, 1981 under theInternational Convention.

FIELD OF THE INVENTION

The present invention relates to a method of detecting the defects in adielectric coating at the surface of an electrically conductiveunderlayer. This method is especially applicable to the detection ofdefects in the coating of submersed pipelines.

BACKGROUND OF THE INVENTION

It is known that these pipelines are provided with double protectionagainst corrosion, one being the protection afforded by a coating layerof plastic material and the other the protection provided by cathodicpolarization of the steel of the pipeline with respect to the sea water,this cathodic polarization being obtained, for instance, by arrangingzinc anodes at given intervals around the pipeline, the polarizationtaking place between the zinc bracelets and the portions of the steelpipe not protected by the coating as a result of the deterioration ofthis coating.

Although it is rather easy to detect and correct coating defectsoccurring before the positioning of the pipeline, no provisions are madeto correct damage suffered during laying of the pipeline. Under theseconditions the zinc bracelets are arranged at intervals based on astatistical evaluation of the defects.

It has been already proposed to control the cathodic protection afterthe positioning of the pipeline, either by measuring the potential ofthe pipe with respect to the surrounding medium, or by measuring theprotection currents circulating either in the pipe or in the surroundingmedium. The measurement of the protection current can not be done duringthe positioning because of the perturbations generated by the parasiticcurrents originating from the numerous electrical machines used duringthe positioning of a pipeline, such as the welding currents.

There are methods already known in electrochemistry to study thecondition of the surface by measuring the impedance of a surfacesubmerged in a conductive medium by using an electrode excited by analternating-current generator of average frequency and apotential-measuring electrode located very closely to the studiedsurface. The impedance is a result of the relation between the measuredpotential and the excitation current delivered. Such a method ofmeasurement is applicable only to an examination taking place very closeto the surface (only a few tens of microns), due to the fact that whenthe distance is of the order of millimeter, the low resistivity of themedium especially when this medium is sea water has a tendency toequalize the potential between the excitation electrode and thesubstratum, so that it is no longer possible to measure the variationsin impedance.

OBJECT OF THE INVENTION

The object of the present invention is to measure the surface impedanceat a distance from the underlayer sufficient to allow relativedisplacement of the underlayer and an excitation electrode, in order tomeasure continuously big and small defects, even if they are close toeach other.

SUMMARY OF THE INVENTION

To accomplish this object the invention provides a method of detectingdefects in a dielectric coating at the surface of an electricallyconductive underlayer, according to which an excitation electrode ispositioned at a distance from said underlayer on the side thereofcovered by said coating, this electrode and this underlayer being placedin a common electrically conductive medium and the potentialscorresponding to said underlayer and said electrode are carried to theterminals of an alternating-current generator. This method ischaracterized by the fact that a catcher coil is oriented essentiallyperpendicular to the electric field lines and an electrode for measuringthe potential is positioned in the opening of said coil and onedetermines the impedance resulting from the voltages and currentsdetected by said measuring electrode and said catcher coil.

The advantage of this method is that it is particularly well suited forthe control of the coating of submerged pipelines during theirpositioning, due to the fact that the high-frequency currents which canbe used are insensitive to the surrounding parasite currents, such aswelding currents, and that the detection can be achieved at a distancefrom the surface of the underlayer sufficient to allow for a continuouscontrol as the positioning of the pipeline progresses when the pipeenters the sea water and leaves the stinger which is an element guidingthe pipe extending at the rear of the barge used for the positioning ofthe pipeline. As a result, this method creates the possibility toachieve control at a point where the pipe has lost all contact with thepipeline positioning installation capable of causing deterioration ofthe protective coating. It is therefore possible by using this method tomeasure the surface of the pipe not protected by the coating of plasticmaterial and to distribute the zinc anode as a function of the requiredprotection by cathodic polarization or to repair the coating of the pipewhen the uncovered surface of the steel passes a certain limit.

BRIEF DESCRIPTION OF THE DRAWING

The accompaying drawing shows schematically the use of the method of theinvention. In the drawing:

FIG. 1 is an elevational view of a barge for positioning of pipelinesshowing the control area of the pipe.

FIG. 2 schematically shows the principles of the detection device.

FIG. 3 schematically shows the application of the method for detectingdefects in the coating of a tube.

FIG. 4 is a graph of the impedance measured with respect to the extentupon the defects and in dependence of the distance between theunderlayer and the catcher coil.

SPECIFIC DESCRIPTION

FIG. 1 simply shows the area Z in which it is desirable to perform thecontrol of the coating of a substratum consisting in this example of apipe 1. One can see that this area is immersed and is situated at theexit end of the stringer S constituting a guiding surface for the pipeand located at the rear of the barge B.

The principle on which the method is based is explained in connectionwith the detection device shown in FIG. 2. This device comprises anexcitation electrode 2 located at a certain distance from the pipe 1 andconnected via a resistance 3 to a terminal of an alternating-currentgenerator 4, whose other terminal is connected to the pipe 1 surroundedby a dielectric coating 5. A toroid catcher coil 6 is located betweenthe excitation electrode 2 and the wall of the pipe 1, essentiallyparallel to the underlayer so that the current lines developing betweenthe excitation electrode 2 and the pipe 1 cross the opening of thetoroid catcher or sensing coil 6. This way the catcher coil measures thecurrent component perpendicular to the pipe. Indeed, if one accepts forinstance a constant current density i having a direction forming anangle with the axis of revolution of the toroid coil, the currentcrossing the opening of this coil is:

    I=I.sub.o cos α

where I_(o) =iS, S being the area of this opening.

Due to the directivity of the measurement of the current by the catchercoil 6 it is possible to obtain significant information regarding theelectrical behavior of the portion of the underlayer surface locatedopposite this coil.

An electrode 7 for measuring the potential is placed in the plane of thecatcher coil 6 in a manner insuring absolute significance of themeasuring of the current. These signals voltage/current are collected bya receiving member 8 which calculates the value of the impedanceexisting between the plan of the torus and the surface of theunderlayer. This directivity of the measuring is precisely the reasonthe method is capable to detect small and large defects located side byside due to the fact that the detecting coil 6 measures only the currentcomponent perpendicular to the surface of the underlayer. Thisdirectivity is further enhanced by making the excitation electrode 2 asbig as possible.

The frequency of the feeding current also influences the sensitivity ofthe current measuring. For instance a frequency of 10,000 Hz feedingferrite-core toroid coils with an initial high permeability allows toattain sensitivities on the range of 10 to 100 mV/mA.

A series of tests have been performed in a simulated sea waterenvironment on a pipe segment coated with a layer of polyethylene havinga thickness of 3 mm whereby certain portions have been eliminated touncover the steel of the pipe. These portions of uncovered surface havea diameter of between 2 and 5 mm, 10×2 cm or extend over a completeportion of the tube. To simulate the real conditions a zinc anode isassociated with the pipe.

In FIG. 3 the electrode 2 is shown to be cylindrical and the toroidalsensing coil 6 part of an array of such coils uniformly spaced about theperiphery of the pipe 1.

                  TABLE    ______________________________________                               Impedance    Area tested    Distance in cm                               (ohms)    ______________________________________    Polyethylene   0.5     cm      9500                   3       cm      8000    S = 0          5       cm      6200                   10      cm      7100    Defect φ 2 mm                   0.5     cm      1100                   3       cm      4700    S = 3.10.sup.-2 cm.sup.2                   10      cm      7000    Defect φ 5 mm                   0.5     cm       350                   3       cm      2400    S = 0.2 cm.sup.2                   10      cm      7000    Defect 2 × 10 cm                   0.5     cm       50                   3       cm       375    S = 20 cm.sup.2                   5       cm       600                   10      cm       750    Uncovered      0.5     cm       20    Surface        3       cm       70    S → ∞                   5       cm       100                   10      cm       180    ______________________________________

Two different diameters for the toroid catcher coil 6 have been tested,namely one of 2.5 cm and the other of 1 cm, each having 200 turns. Usinga 10 kHz current the measured sensitivities are respectively 45 and 40mV/mA. The excitation electrode is fed by a current limited to 150 mA.

The measurements have been taken over the entire length of the pipe, incontact with the coating at a distance of approximately 5 mm from theunderlayer, respectively at 30, 50 and 100 mm. The above table shows theresults obtained with the toroid catcher coil having a diameter of 2.5cm.

The diagram in FIG. 4 gives a graphical representation of these resultswhereby the distance between the catcher coil 6 and the underlayer isthe parameter.

One can see that at a distance of 5 mm defects in the range of 10⁻² cm²are detectable, at 3 cm defects of 10⁻¹ cm² are still detectable, at 5cm it is possible to detect defects over 1 cm², while at 10 cm the limitlies between 5 and 10 cm².

FIG. 3 represents an application of the method according to theinvention in the detection of the coating of a pipeline immersed in seawater. This device corresponds exactly to the schematic representationof the principle in FIG. 2, a series of toroid catcher coils 6 beingdistributed annularly around the pipe 1 and connected to a multichannelreceiving element 8 as well as the potential measuring electrodes 7located in the plan of each catcher coil. The excitation electrode 2comprises a ring connected to one of the terminals of the alternativecurrent generator 4 whose other terminal is connected to the pipe 1. Ithas to be noted that this detection method is also applicable to thedetection of defects of pipelines after their positioning in the sea. Inthis case, instead of connecting the source of the alternative current 4to the duct 1 as this can be easily done during positioning, the secondterminal of the generator 4 can be connected to an electrode 9 locatedvery far from the pipe 1, at a distance 50 to 100 times bigger than thatbetween the excitation electrode 2 and the pipe 1 which permits toobtain practically the same results as by bringing the pipe directly tothe potential of the mass of the alternative current generator.

It is known that in a good number of cases the immersed pipelines arecovered with a layer of concrete in order to give a weight superior tothe ascending pressure of the displaced volume of water. Tests have beenperformed to evaluate the influence of the concrete layer on themeasuring and especially the influence of the resistivity of theconcrete. The resistivity of the concrete can vary widely depending onits composition, preparation and age and on its water content. Theresistivity in bulk of the concrete 10 days after its preparation liesbetween 5000 ohms.cm and 0.5 megohms.cm. After 90 days the resistivitycan reach values in the range of 500 megohms.cm and the resistivity offurnace-dried concrete can increase to 10⁶ megohms.cm.

But in the case of immersed pipelines, the resistivity depends on thewater absorption. Test have shown that the resistivity decreases byapproximately 1 megohm.cm to 10,000 ohms.cm a few hours after the samplehas been immersed in the simulated sea water. These values have to becompared with the resistivity of the sea water which is in the range of30 ohms.cm. However, the steel reinforcement renders the situation morecomplex.

Various measuring tests have been carried out with steel tubes coatedwith polyethylene as aforementioned and with a 5 cm layer of concretewith or without reinforcement by a grating of steel wire of 1.6 mm withmesh sizes of 2.5×2.5 cm. The concrete was prepared in a proportion of450 Portland cement to 1 m³ sand (0-11) and 350 kg Portland cement to 1m³ sand (0-20). The simulated defects were obtained by perforations of 6to 12 mm in diameter.

The measuring was carried out in the immediate vicinity of the concretelayer or at 5 to 6 cm from the steel pipe. The impedances measured withdefects in the concrete layer as well as in the dielectric adjacent tothe pipe are in the same range as those measured without concrete at thesame distance from the steel pipe.

We claim:
 1. A method of detecting defects in a dielectric coating onthe surface of electrically conductive pipe adapted to serve as asubmerged pipeline, comprising the steps of:feeding said pipesubstantially continuously below the surface of a body of water througha submerged zone above a floor of said body of water and below thesurface of said body of water; spacedly juxtaposing said pipe with anexcitation electrode extending only along said zone with said pipe insaid body of water so that said pipe moves progressively past saidexcitation electrode through said zone; applying an alternating currentacross said pipe and said excitation electrode; disposing between saidelectrode and said pipe in said body of water at least one ferrite-coretoroidal current sensing coil with a plane of said coil substantiallyperpendicular to a perpendicular from said surface of said pipe wherebysaid coil measures electric current passing substantially along saidperpendicularly between said pipe and said electrode; providing apotential detecting electrode in an opening of said coil; andcalculating the impedance of an electric current path between saidsurface of said pipe and said excitation electrode in response to thecurrent measured by said coil and electrical potential detected by saidpotential detection electrode.
 2. The method defined in claim 1 whereinsaid pipe is passed through said excitation electrode which is tubularand surrounds said pipe in said zone, and a plurality of such coilsdisposed around said pipe are provided in said zone between saidexcitation electrode and said pipe.